1. Technical Field
The present invention relates in general to an improved data storage system and in particular to an improved method and system for validation of an acquisition burst sequence which precedes or follows a data block in a data storage system. Still more particularly, the present invention relates to an improved method and system for detection and validation of an acquisition burst sequence which is tolerant of increased track skew.
2. Description of the Related Art
Modern data processing systems often use digital signal recording devices attached to host processors to record records as addressable units within magnetic tape storage systems.
Examples of systems which may be utilized to record records within a magnetic tape storage system are disclosed within Milligan et al., U.S. Pat. No. 4,393,445; Milligan et al., U.S. Pat. No. 4,435,762; Videki II, U.S. Pat. No. 4,471,457; Cole et al., U.S. Pat. No. 4,603,382; Bauer et al., U.S. Pat. No. 4,423,480; and Fry et al., U.S. Pat. No. 4,403,286. Each of the aforementioned patents discloses a magnetic tape storage system which may be advantageously employed in carrying out the method and system of the present invention.
In such data storage systems, it is practically a necessity that each track of data within the recording medium includes multiple synchronization characters. Such synchronization characters are necessary so that the data may be considered self-synchronizing. Without such self-synchronizing, data may not be successfully recovered. This self-synchronizing is typically accomplished by inserting various synchronization characters between small blocks or sets of data signals so that the data stored therein may be accurately and efficiently recovered.
In modern data storage systems, data are typically written to multiple tracks simultaneously. When such multiple track recording is utilized, it is possible to determine various necessary parameters which may affect data recovery, even though individual track data within a group of multiple tracks may be lost. One such parameter is the ending or beginning of a data block within a plurality of data blocks. Typically, an interblock gap character is written between adjacent data blocks and an acquisition burst sequence is then written immediately following and preceding each interblock gap.
The acquisition burst sequence typically comprises the highest frequency pattern permissible within the Run-Length-Limited (RLL) code which is employed in a particular system and such acquisition burst sequences then make up the first and last portion of each formatted data block. The acquisition burst sequence is utilized to provide a constant stream of pulses from the read detector to clock acquisition circuits during a Read-While-Write (RWW) or a Read Forward operation. Once the track logic detects a transition from an interblock gap to an acquisition burst sequence, a Phase-Locked-Loop (PLL) circuit may be placed in an acquisition mode for a predetermined period of time in order to permit a system clock to lock onto the frequency of the data pulses.
In bidirectional data storage systems, the ending burst sequence is typically the reverse of the acquisition burst sequence and is utilized for the same function as the acquisition burst sequence during a Read Backward operation. It should therefore be apparent that the integrity of the acquisition burst sequence pattern during the time a Phase-Locked-Loop (PLL) circuit is in an acquisition mode is highly important to the ability of a system clock to correctly lock onto an incoming signal. If one or more tracks within a multitrack system fail to detect a valid acquisition burst sequence pattern during this short interval there exists the possibility that the clock may not acquire frequency synchronization and the data flow will not correctly detect beginning synchronization signals which follow the acquisition burst sequence.
This problem becomes increasingly difficult in storage formats which permit large track skew across a tape and which employ a global clock derived from the outputs of multiple tracks. As skew increases as a result of interchange of storage media, the portion of the acquisition burst sequence which is utilized to acquire a lock for the Phase-Locked-Loop circuit when written will not be identical to that utilized in a subsequent read operation within another system. This occurs as a result of the fact that the maximum edge-to-edge track skew may have doubled and the clock cannot start clock acquisition until the track logic has determined that all tracks have entered the acquisition burst sequence. Thus, the greater the track skew the more difficult validation of an acquisition burst sequence becomes since the detection of an acquisition burst sequence will occur further into the acquisition burst sequence of the early tracks in an area of the acquisition burst sequence not previously examined during the writing of the burst for acceptable quality. As a result, the readability of a data block which has just been written will be degraded if that portion of the acquisition burst sequence which occurs following the nominal acquisition interval is defective.
It should therefore be obvious that a need exists for a method and system wherein an acquisition burst sequence may be successfully validated despite increased track skew without requiring a perfect acquisition burst sequence for each track.